34 research outputs found
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
Strange Quark Contributions to Parity-Violating Asymmetries in the Forward G0 Electron-Proton Scattering Experiment
We have measured parity-violating asymmetries in elastic electron-proton
scattering over the range of momentum transfers 0.12 < Q^2 < 1.0 GeV^2. These
asymmetries, arising from interference of the electromagnetic and neutral weak
interactions, are sensitive to strange quark contributions to the currents of
the proton. The measurements were made at JLab using a toroidal spectrometer to
detect the recoiling protons from a liquid hydrogen target. The results
indicate non-zero, Q^2 dependent, strange quark contributions and provide new
information beyond that obtained in previous experiments.Comment: 5 pages, 2 figure
The G0 Experiment: Apparatus for Parity-Violating Electron Scattering Measurements at Forward and Backward Angles
In the G0 experiment, performed at Jefferson Lab, the parity-violating
elastic scattering of electrons from protons and quasi-elastic scattering from
deuterons is measured in order to determine the neutral weak currents of the
nucleon. Asymmetries as small as 1 part per million in the scattering of a
polarized electron beam are determined using a dedicated apparatus. It consists
of specialized beam-monitoring and control systems, a cryogenic hydrogen (or
deuterium) target, and a superconducting, toroidal magnetic spectrometer
equipped with plastic scintillation and aerogel Cerenkov detectors, as well as
fast readout electronics for the measurement of individual events. The overall
design and performance of this experimental system is discussed.Comment: Submitted to Nuclear Instruments and Method
Transverse Beam Spin Asymmetries in Forward-Angle Elastic Electron-Proton Scattering
We have measured the beam-normal single-spin asymmetry in elastic scattering
of transversely-polarized 3 GeV electrons from unpolarized protons at Q^2 =
0.15, 0.25 (GeV/c)^2. The results are inconsistent with calculations solely
using the elastic nucleon intermediate state, and generally agree with
calculations with significant inelastic hadronic intermediate state
contributions. A_n provides a direct probe of the imaginary component of the
2-gamma exchange amplitude, the complete description of which is important in
the interpretation of data from precision electron-scattering experiments.Comment: 5 pages, 3 figures, submitted to Physical Review Letters; shortened
to meet PRL length limit, clarified some text after referee's comment
X-Ray Polarization of Solar Flares Measured with Rhessi
The degree of linear polarization in solar flares has not yet been precisely
determined despite multiple attempts to measure it with different missions. The
high energy range in particular has very rarely been explored, due to its
greater instrumental difficulties. We approached the subject using the Reuven
Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study 6 X-class
and 1 M-class flares in the energy range between 100 keV and 350 keV. Using
RHESSI as a polarimeter requires the application of strict cuts to the event
list in order to extract those photons that are Compton scattered between two
detectors. Our measurements show polarization values between 2% and 54%, with
errors ranging from 10% to 26% in 1 sigma level. In view of the large
uncertainties in both the magnitude and direction of the polarization vector,
the results can only reject source models with extreme properties.Comment: 26 pages, 11 figures, accepted for publication by Solar Physic
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
Strange Quark Contributions to Parity-Violating Asymmetries in the Forward G0 Electron-Proton Scattering Experiment
We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12 ≤ Q2 ≤ 1.0 GeV2. These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange quark contributions to the currents of the proton. The measurements were made at JLab using a toroidal spectrom- eter to detect the recoiling protons from a liquid hydrogen target. The results indicate non-zero, Q2 dependent, strange quark contributions and provide new information beyond that obtained in previous experiments
Rotavirus group : a genotype circulation patterns across Kenya before and after nationwide vaccine introduction, 2010-2018
Background
Kenya introduced the monovalent G1P [8] Rotarix® vaccine into the infant immunization schedule in July 2014. We examined trends in rotavirus group A (RVA) genotype distribution pre- (January 2010–June 2014) and post- (July 2014–December 2018) RVA vaccine introduction.
Methods
Stool samples were collected from children aged < 13 years from four surveillance sites across Kenya: Kilifi County Hospital, Tabitha Clinic Nairobi, Lwak Mission Hospital, and Siaya County Referral Hospital (children aged < 5 years only). Samples were screened for RVA using enzyme linked immunosorbent assay (ELISA) and VP7 and VP4 genes sequenced to infer genotypes.
Results
We genotyped 614 samples in pre-vaccine and 261 in post-vaccine introduction periods. During the pre-vaccine introduction period, the most frequent RVA genotypes were G1P [8] (45.8%), G8P [4] (15.8%), G9P [8] (13.2%), G2P [4] (7.0%) and G3P [6] (3.1%). In the post-vaccine introduction period, the most frequent genotypes were G1P [8] (52.1%), G2P [4] (20.7%) and G3P [8] (16.1%). Predominant genotypes varied by year and site in both pre and post-vaccine periods. Temporal genotype patterns showed an increase in prevalence of vaccine heterotypic genotypes, such as the commonly DS-1-like G2P [4] (7.0 to 20.7%, P < .001) and G3P [8] (1.3 to 16.1%, P < .001) genotypes in the post-vaccine introduction period. Additionally, we observed a decline in prevalence of genotypes G8P [4] (15.8 to 0.4%, P < .001) and G9P [8] (13.2 to 5.4%, P < .001) in the post-vaccine introduction period. Phylogenetic analysis of genotype G1P [8], revealed circulation of strains of lineages G1-I, G1-II and P [8]-1, P [8]-III and P [8]-IV. Considerable genetic diversity was observed between the pre and post-vaccine strains, evidenced by distinct clusters.
Conclusion
Genotype prevalence varied from before to after vaccine introduction. Such observations emphasize the need for long-term surveillance to monitor vaccine impact. These changes may represent natural secular variation or possible immuno-epidemiological changes arising from the introduction of the vaccine. Full genome sequencing could provide insights into post-vaccine evolutionary pressures and antigenic diversity
Dynamics of the O(e,e'p) cross section at high missing energies
We measured the cross section and response functions (R_L, R_T, and R_LT) for the 16O(e,e'p) reaction in quasielastic kinematics for missing energies 25 60 MeV and P_miss > 200 MeV/c, the cross section is relatively constant. Calculations which include contributions from pion exchange currents, isobar currents and short-range correlations account for the shape and the transversity but only for half of the magnitude of the measured cross section